1. Field of the Invention
The invention relates to a method for etching a recrystallized aluminum foil for electrolytic capacitors by carrying out an electrolytic tunnel etching process in a first etching stage.
2. Description of the Prior Art
Such method of etching is known from German Pat. No. 28 01 218. In the first etching stage, a tunnel structure is made in a chloride-containing solution where the etched tunnels grow auto-catalytically in a crystal-oriented [100] direction into the recrystallized aluminum lattice at a growth rate of several .mu.m/s with a pore diameter of about 0.2 .mu.m. The enlargement of the etched tunnel in the further etching is accomplished by the known method which is likewise electrolytically, in a chloride-ion-containing solution. An etching current density corresponding to the potential range below the pitting potential of aluminum is used. The maximally usable and relatively low etching current density is determined by the passive current density of aluminum in the respective solution. If the etching current density referred to the actual roughened foil surface is greater than the maximum passive current density, coarse pitting is produced. In expanding the tunnels electrolytically, the danger therefore exists that the tunnel structure is destroyed entirely or partially by coarse pitting, if the current density distribution is irregular due to the arrangement of contact rolls, and in the case of foils with irregular or altogether low tunnel density (corresponding to a small surface). Furthermore, in electrolytic etching, a high amount of etching charge is needed, which means high power consumption and, a considerable amount of equipment for the large currents that must be made available.